1. Field of Invention
This invention relates to lubricants for die lubrication and a manufacturing method for an iron-based powder compact for powder metallurgy.
2. Description of Related Art
In general, iron-based powder compacts for powder metallurgy are manufactured by the steps of mixing an iron-based powder, an alloy powder, for example, a copper powder and a graphite powder, and furthermore, a lubricant, for example, zinc stearate and lead stearate, to prepare an iron-based mixed powder; filling a die with the iron-based mixed powder; and performing pressure molding. Densities of the resulting compacts are generally 6.6 to 7.1 Mg/m3.
These iron-based powder compacts are subjected to sintering to make sintered materials, and are further subjected to sizing and cutting as necessary to make powder metallurgy products. In cases in which further increased strength is required, a carburization heat-treatment, or a bright heat-treatment, may be performed after completion of the sintering.
By using this powder metallurgy technique, it has become possible to produce nearly final shape, that is, “near net shape”, complicatedly shaped components with high dimensional accuracy by one-time molding without many steps of cutting works. Therefore, it has become possible to decrease cutting costs to a great extent as compared to conventional manufacturing methods. As a consequence, iron-based powder metallurgy products were used as components of automobiles in an amount exceeding 6 kg per automobile in 1998 in Japan. Recently, it is strongly required of iron-based powder metallurgy products that there be a further improvement in dimensional accuracy in order to decrease costs by omitting the cutting works and that there be an increase in strength in order to produce miniaturized and lightweight components.
In order to increase the strength of powder metallurgy products (sintered components), it is effective to increase the density of sintered components by increasing the density of compacts. Accompanying the increase in the density of sintered components, cavities in the components are decreased, and mechanical properties, for example, tensile strength, impact value, and fatigue strength are improved.
As a compaction method capable of increasing the density of iron-based powder compacts, the twice compaction and twice sintering method, in which iron-based mixed powder is subjected to ordinary compaction and sintering, and thereafter, is subjected to another compaction and sintering, and the sintering and forging method, in which after once compacting and once sintering are performed, hot forging is performed, have been suggested.
Furthermore, for example, the warm compaction technique, in which metallic powders are molded while being heated is disclosed in Japanese Unexamined Patent Application Publication No. 2-156002, Japanese Examined Patent Application Publication No. 7-103404 and U.S. Pat. Nos. 5,256,185 and 5,368,630. This warm compaction technique is intended to decrease frictional resistance between the particles and between the compact and the die, and to improve the compactibility by a portion of, or by the entirety of the lubricant, being fused during the warm molding, and thereby, being uniformly dispersed between the powder particles. It is believed that this warm compaction technique has the most advantageous cost among the above-mentioned manufacturing methods for high-density compacts. According to this warm compaction technique, an iron-based mixed powder prepared by blending 0.5% by weight of graphite and 0.6% by weight of lubricant to Fe-4Ni-0.5Mo-1.5Cu partially alloyed iron powder can be molded at 130° C. and at a pressure of 7 t/cm2 (686 MPa) to produce a compact having a density of about 7.30 Mg/m3.
According to the techniques described in Japanese Unexamined Patent Application Publication No. 2-156002, Japanese Examined Patent Application Publication No. 7-103404 and U.S. Pat. Nos. 5,256,185 and 5,368,630, however, because the fluidity of the powder mixture is insufficient, there have been problems in that the productivity is decreased, unevenness occurs in the density of the compact, and the properties of the sintered material fluctuate. Furthermore, there have been problems in that because a large ejection force is required during compacting, flaws are generated at the surface of the compact, and the lifetime of the die is decreased.
Furthermore, in these warm compaction techniques, the lubricant is contained in the iron-based mixed powder in order to decrease frictional resistance between the particles and between the compact and the die and to improve the compactibility. A part of, or the entirety of, the lubricant is, however, fused during the warm compaction so as to be pushed out to the vicinity of the surface of the compact. During the subsequent sintering, the lubricant is pyrolyzed or vaporized and dissipated from the compact and coarse cavities are formed in the vicinity of the surface of the sintered material. Therefore, there has been a problem that the mechanical strength of the sintered material is decreased.
In order to solve this problem, in Japanese Unexamined Patent Application Publication No. 8-100203, a technique in which in ordinary temperature compaction or in warm compaction, the surface of the die is coated with an electrified lubricant powder to decrease the amount of the lubricant in the iron-based mixed powder and to achieve a high-density compact. According to this method, however, because only one kind of lubricant for die lubrication is applied by coating, the shape of the lubricant changes near its melting point so that the function of lubricating changes to a great extent. As a consequence, there has been a problem in that the range of the compacting temperature is restricted by the melting point of the lubricant Even when the surface of the die is coated with a lubricant for die lubrication to decrease the amount of the lubricant in the iron-based mixed powder, there is still a problem that some components of the mixed lubricant cannot exhibit the effect of lubricating due to the decrease in the amount and an increase in green density is not achieved.
Commercially available lubricants for die lubrication are intended for use at room temperature. Therefore, when these commercially available lubricants for die lubrication are adhered by electrification to preheated dies, there are problems that the lubricants may be completely fused on the surface of the dies and not uniformly adhered, and the lubricants are likely to move during the compaction pressure, such that the compact and the surface of the dies may be directly contacted so as to increase the ejection force.
Accordingly, there is a strong demand for an ordinary temperature compaction technique in which high-density compacts can be produced by one time compacting at room temperature. As the above-mentioned ordinary temperature compaction technique, a molding technique using die lubrication has been attempted as described, for example, in W. G. Ball et al., The International Journal of Powder Metallurgy, APMI International, vol.33, No.1, 1997, pp.23-30. In the case in which the die is coated with a commercially available lubricant for die lubrication using a conventional die lubrication apparatus, however, the lubricant is not uniformly dispersed and adhered to the surface (wall) of the die with a high degree of reproducibility even at room temperature. Consequently, this technique has not yet become industrially practical.
In addition, from the viewpoint of increasing the strength of automobile components, and from the viewpoint of cost, there has been a demand for developing a manufacturing method for a high-density iron-based powder compact that can produce a compact having higher density can be produced by one time compacting.